1. Field of the Invention
The present invention relates to limited slip differentials, and more particularly, to the retention of the electromagnet in a limited slip differential having a electromagnetically actuated clutch.
2. Description of the Related Art
Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts.
The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a "spin out" of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, operatively located between the rotating case and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque can always be applied to a wheel having traction, e.g., a wheel located on dry pavement. The initial torque generates gear separating forces between the first pinion gears and the side gears intermeshed therewith. The gear separating forces urge the two side gears outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches are disclosed in U.S. Pat Nos. 4,612,825 (Engle), 5,226,861 (Engle), 5,556,344 (Fox), and U.S. patent application Ser. No. 09/030,602, filed Feb. 25, 1998, each of which are assigned to the assignee of the present invention and expressly incorporated herein by reference.
Certain prior art limited slip differentials provide, between the first of the two side gears and its associated clutch element, interacting camming portions having ramp surfaces. In response to an initiating force, this clutch element is moved towards and into contact with the surface against which it frictionally engages, which may be a mating clutch element attached to the casing, or an interior surface of the casing itself, as the case may be, thereby axially separating the clutch element and its adjacent first side gear, the ramp surfaces of their interacting camming portions slidably engaging, the rotational speed of the clutch element beginning to match that of the differential casing due to the frictional engagement. Relative rotational movement between the ramp surfaces induces further axial separation of the clutch element and the first side gear. Because the clutch element is already in abutting contact with the surface against which it frictionally engages, the first side gear is forced axially away from the clutch element by the camming portions.
Certain embodiments of limited slip differentials utilize an electromagnet to actuate the clutch. The differential casing, in which the clutch is disposed, rotates within the housing and is rotatably supported by a pair of bearings. The electromagnet, which actuates the clutch, is mounted in fixed relationship to the axle housing and is rotatably supported on the differential casing by a separate bearing. An example of a prior electrically actuated limited slip differential is disclosed in allowed U.S. patent application Ser. No. 09/030,602.
FIG. 1 depicts an embodiment of prior axle assembly 10 having electrically actuated limited slip differential assembly 12. Axle assembly 10 may be a conventional axle assembly or comprise part of a transaxle assembly. Therefore, it is to be understood that the term "axle assembly" encompasses both conventional (rear wheel drive) axle assemblies as well as transaxle assemblies. Differential assembly 12 comprises electromagnet 14, rotatable casing 16 constructed of joined first and second casing parts 16a and 16b, respectively, and providing inner cavity 18, which is defined by the interior surface of the circumferential wall portion of first casing part 16a and end wall portions 20, 22 of first and second casing parts 16a, 16b, respectively. Disposed within cavity 18 are side gears 24, 26 and pinion gears 28, 30. The teeth of the side gears and pinion gears are intermeshed, as shown. Pinion gears 28, 30 are rotatably disposed upon cylindrical steel cross pin 32, which extends along axis 34. The ends of cross pin 32 are received in holes 36, 38 diametrically located in the circumferential wall of casing part 16a.
Axles 40, 42 are received through hubs 44, 46, respectively formed in casing end wall portions 20, 22, along common axis of rotation 48, which intersects and is perpendicular to axis 34. Axles 40, 42 are respectively provided with splined portions 50, 52, which are received in splines 54, 56 of side gears 24, 26, thereby rotatably fixing the side gears to the axles. The axles are provided with circumferential grooves 58, 60 in which are disposed C-rings 62, 64, which prevent the axles from being removed axially from their associated side gears. The terminal ends of the axles may abut against the cylindrical surface of cross pin 32, thereby restricting the axles' movement toward each other along axis 48.
Clutch element 66 is attached to side gear 24 and rotates therewith. Clutch element 66 is of the cone clutch variety and has frustoconical surface 68 which is adjacent to, and clutchedly interfaces with, complementary surface 70 provided on the interior of casing part 16a. Clutch element 72 is also of the cone clutch variety and has frustoconical surface 74 which is adjacent to, and clutchedly interfaces with, complementary surface 76 also provided on the interior of casing part 16a.
Disposed between cone clutch element 72 and side gear 26 is annular cam plate 78, which abuts thrust washer 82 adjacent end wall portion 22. First ball and ramp arrangement 84, 86, 92 is comprised of a first plurality of paired spiral slots 84, 86 located in cam plate 78 and primary cone clutch element 72, respectively. Slots 84, 86 define a helically ramping path followed by ball 92, which may be steel, disposed in each slot pair and a first ramp angle. With electromagnet 14 de-energized, balls 92 are seated in the deepest portion of slots 84, 86 by Belleville spring 94. The actuation sequence is created by the momentary difference in rotational speed between cone clutch element 72 and cam plate 78 as frustoconical surfaces 74 and 76 seat against each other.
Second ball and ramp arrangement 104, 106, 108 is comprised of a second plurality of paired spiral slots 106, 108 located in side gear 26 and cam plate 78, respectively. With electromagnet 14 de-energized, balls 104 are urged into the deepest portion of slots 106, 108 by Belleville spring 94. Each pair of slots 106, 108 defines a helically ramping path followed by ball 104, which may be steel, disposed in the slot pair and a second ramp angle. The second ramp angle is substantially less than first ramp angle. That second ramp angle is "shallower" than the first ramp angle means that ball 104 is able to transfer greater axially directed loads than ball 92. Thus, ball 104 is substantially larger in diameter than ball 92, providing a greater contact area with slots 106, 108 than ball 92 has with slots 84, 86, maintaining stresses associated with the higher loads at acceptable levels. As will be further described below, ball and ramp arrangement 104, 106, 108 transfers axial forces between cam plate 78, which abuts thrust washer 82 at end wall portion 22, and side gear 26, which communicates with transfer block 114, side gear 24, secondary cone clutch element 66 and surface 76 of casing part 16a. A more detailed discussion of the ball and ramp arrangements is disclosed in allowed U.S. patent application Ser. No. 09/030,602.
In operation, a variable coil current on electromagnet 14 induces a variable amount of magnetic clamping force between casing part 16a and cone clutch element 72, which induces a variable amount of torque to be exerted by casing part 16a on element 72. As electromagnet 14 is activated, axial separation of primary cone clutch element 72 and cam plate 78 is induced as cone clutch element 72 is magnetically pulled to the left against the force of Belleville spring 94 into clutched engagement with casing part 16a through frustoconical surfaces 74 and 76. In response to the initial flow of magnetic flux, cone clutch element 72 is pulled to the left and surfaces 74 and 76 abut, entering frictional engagement. As cone clutch element 72 and cam plate 78 separate axially, ball 92 is caused to rotate along the ramping helical paths of slots 84, 86 due to the relative rotation between element 72 and cam plate 78. Cam plate 78 is urged against thrust washer 82 by the force of Belleville spring 94 and gear separation forces between pinion gears 28, 30 and side gear 26. As ball 92 rotates further along the helical ramp paths, frustoconical surfaces 74, 76 are forced into tighter frictional engagement and cam plate 78, still abutting thrust washer 82, reaches the end of its rotational travel relative to cone clutch member 72.
Once cam plate 78 reaches its end of travel relative to cone clutch member 72, side gear 26 begins to rotate relative to cam plate 78. Relative rotation of side gear 26 and cam plate 78 causes ball 104 to rotate along the ramping helical paths of slots 106, 108, which axially follows the centerline of ball 104, from surfaces 110, 112. Side gear 26 moves towards the right, forcing cone clutch element 66 into abutment with casing part 16a via transfer block 114 and side gear 24 in the manner described above. As surfaces 68, 70 engage, side gear 26 reaches its end of travel, rotationally and axially, relative to cam plate 78. As ball 104 becomes more tightly compressed between slots 106, 108, force is transferred along a lines between end wall portion 22, thrust washer 82, cam plate 78, ball 104, side gear 26, transfer block 114, side gear 24, cone clutch member 66 and casing part 16a. Because the ramp angle of slots 106, 108 is smaller than the ramp angle of slots 84, 86, a greater engagement force is exerted on cone clutch element 66 than on cone clutch element 72. It is estimated that 80 percent of the total torque transfer between casing 16 and axles 40, 42 is provided by the engagement of clutch surfaces 68, 70, and only 20 percent by the engagement of clutch surfaces 74, 76.
Transfer block element 114, which may be steel, is disposed about cross pin 32 and adapted to move laterally relative thereto along axis 48 to transfer movement of side gear 26 to side gear 24, thereby engaging clutch element 66. Transfer block element 114 is attached directly to cross pin 32 by means of spring pin 116. Spring pin 116, which comprises a rolled sheet of spring steel, extends through centrally-located cross bore (not shown) which extends perpendicularly to axis 34 through cross pin 32. Spring pin 116 is retained in cross bore by means of an interference fit. The shear loads associated with torque transmission are exerted on cross pin 32 near its opposite ends, particularly between the circumferential wall of casing part 16a and the adjacent pinion gears 28, 30. At the longitudinal center of cross pin 32, where cross bore is located, there is no substantial shear stress exerted on cross pin 32.
Transfer block element 114 includes opposite bearing sides 118, 120 for transferring movement of side gear 26 to side gear 24, as described above, and allows terminal ends 122, 124 of axles 40, 42, respectively, to abut the cylindrical side surface of cross pin 32. Surfaces 126, 128 of transfer block element 114 abut pinion gears 28, 30, respectively, as in differential assembly 12, thereby restricting movement of the transfer block element, and thus cross pin 32, relative to casing 16 along axis 34. Thus it will be understood that transfer block 114 serves as a cross pin retention element. Transfer block 114 moves laterally relative to cross pin 32, along axis 48, such that rightward movement of side gear 26, described above, is transferred to side gear 24. Surface 118 of transfer block 114 is brought into abutting contact with the surface of side gear 24. Thus, during actuation of electromagnet 14, side gear 26 is urged rightward, as viewed in FIG. 1, into abutting contact with transfer block element 114. Transfer block element 114 moves rightward, into abutting contact with side gear 24; and side gear 24 moves rightward, urging surface 68 of clutch element 66 into frictional engagement with surface 70 of case part 16a, thereby providing additional torque transfer capacity to the differential than would otherwise be provided with single cone clutch element 72.
Provided on the exterior surface of casing part 16a is flange 134, to which ring gear 136 is attached. The teeth of ring gear 136 are in meshed engagement with the teeth of pinion gear 137 which is rotatably driven by an engine (not shown), thus rotating differential case 16 within axle housing 138. As casing 16 rotates, the sides of holes 36, 38 bear against the portions of the cylindrical surface of cross pin 32 in the holes. The rotation of cross pin 32 about axis 48 causes pinion gears 28, 30 to revolve about axis 48. The revolution of the pinion gears about axis 48 causes at least one of side gears 24, 26 to rotate about axis 48, thus causing at least one of axles 40, 42 to rotate about axis 48. Engagement of the clutch arrests relative rotation between the side gears and the differential casing.
Differential casing 16 is rotatably supported within axle housing 138 by means of first and second bearings 140, 142. Bearings 140, 142 each include cup portion 144, 146 and cone and roller portion 148, 150 each having cone 174, 176 and plurality of rollers 178, 180. Cup portions 144, 146 are disposed within recesses 152, 154 of axle housing 138, respectfully. Cone and roller portions 148, 150 are attached to first axially extending shoulders 156, 158 of casing portion 16b. Spacers 160 are provided axially between axle housing 138 and bearings 140, 142. Electromagnet 14 is rotatably supported on second differential casing portion 16b by bearing 162. Bearing 162 is attached to second axially extending shoulder 164 of casing portion 16b and recess 166 of electromagnet 14.
Electromagnet 14 is rotatably fixed relative to axle housing 138 and disposed in close proximity to casing 16, which rotates relative thereto. The voltage applied to electromagnet 14 to energize same and actuate clutch 72 may be controlled by a control system (not shown) which is in communication with sensors (not shown) which indicate, for example, excessive relative rotation between axles 40, 42, and thus the need for traction control. Housing 138 includes hole 168 fitted with rubber grommet 170 through which extend leads 172. Through leads 172 the control system provides voltage to electromagnet 14. As electromagnet 14 is energized, a magnetic initiating force is applied to clutch element 72 by a toroidal electromagnetic flux path (not shown) which is established about the annular electromagnet; the flux path flows through ferrous casing portions 16a and 16b and through clutch element 72. Clutch element 72 is thus magnetically drawn into engagement with casing 16 during operation of electromagnet 14.
One way to reduce the cost and improve the reliability of an axle assembly is to reduce the number of components parts. Reducing the number of bearings may reduce the cost of material, the cost of assembly labor, and the number of moving parts, thereby improving durability and reliability.
Thus, it is desirable to reduce the number of bearings in an axle assembly.